1. Field of the Invention
This invention relates to organometallic-based luminescent polymers.
2. Description of Related Technology
Luminescent metal complexes incorporated into a polymeric structure should have significantly improved EL properties, because diffusion of ions and crystallization of the luminescent layer are hindered by the polymeric phosphor. This leads to a far higher stability of the device structure. Furthermore, this new type of metal monomer complex can be easily patterned by imagewise exposure to actinic radiation (UV light, visible light, electron beams, or X-rays).
In general, there are four different groups of metal ion containing polymers:
(1) Coordination polymers and poly(electrolytes),
(2) Polymers containing neutral polar groups doped with metal complexes,
(3) Polymer-metal-complexes, prepared by deprotonation of the polymer and subsequent reaction with metal salts, and
(4) Metal-polymer complexes, prepared by polymerisation of the metal monomer complex
Most of these groups have significant disadvantages, which hinder application in EL devices. Thus coordination polymers tend to be insoluble in organic solvents due to their salt-like structure. Doping of polymers, e.g. epoxy resin with lanthanide diketonates, poly(propylene oxide) with lanthanide halides, poly(ethylene glycol) with metal nitrates, or polymer based crown ethers with Eu(II) salts, gives undefined mixtures with poor metal co-ordination environments, leading to poor luminescence properties. Another drawback is the electronically insulating character of this type of polymer, which makes charge injection very difficult. The largest group of metal-polymer-complexes consists of polymers containing acidic or β-diketonate groups, which are easily deprotonated to form strong ionic bonds with the lanthanide or aluminium ion. Well known complexes include carboxylic acid homo- and co-polymers, e.g. poly(styrene acrylic acid), poly(dicarboxylic acid styrene), poly(diketonate), and poly(arylene ether) containing 8-hydroxyquinoline in the side chain as well as doped poly(pyridine). The main disadvantage here is that usually only low doping levels can be achieved (generally 10 wt % at maximum).
To avoid this major drawback, the fourth approach has been used. Metal complexes have been prepared from ligands that contain polymerisable groups, e.g. acrylate. Polymerisation of the monomeric metal complex gives a polymer with a defined structure. These types of compounds have the potential to possess excellent properties in EL devices, combining the advantages of polymers with the emission characteristics of the chosen organometallic complex, for example the colour purity of lanthanide emission. Examples of this class of polymer are poly(lanthanide methacrylate) and poly(lanthanide octanoate), but these are not luminescent. Heteroleptic acrylate complexes with photo-sensitising ligands, e.g. β-diketonate, salicylate, and naphthoate, and their copolymerisation with methylmethacrylate and styrene have been reported. An EL device using this type of material was highly inefficient.
However the use of such luminescent lanthanide acrylate monomers gives rise to disadvantages. In particular homopolymerisation is impossible due to steric hindrance. Therefore the lanthanide monomers have been copolymerised with methylmethacrylate or styrene. This gives an insulating polymer with a low lanthanide content. Other disadvantages include the very low solubility of acrylate complexes in non-coordinating solvents and the very high toxicity of acrylic acid and similar monomers with high vapour pressure.